1. Field of the Invention
In general, this invention relates to hard disk drives. More particularly, it relates to a drive having a system that controls operation of the drive upon detecting that an off-track condition has been caused by a physical shock.
2. Description of the Related Art
Increasingly strict standards have been developing for judging whether a magnetic hard disk drive performs acceptably in an environment entailing physical shocks. A major impetus for such stricter standards is the increasingly hostile environment in which such a drive is used. The environment to which such a drive in a personal computer ("PC") is subjected is generally far less controlled than that for a drive for a mainframe or minicomputer. A drive in a PC (whether a desktop, laptop, or smaller PC) can be operating when it is subjected to physical shock resulting for example from a person bumping into the computer or dropping something on it.
An off-track condition is characterized by a head being displaced radially from the centerline of a track a sufficient distance so as not to be able to read or write reliably with respect to the track. Such an off-track condition can be caused by a physical shock. A shock-caused off-track condition is somewhat of a problem when it occurs during a read operation, because it can prevent the head from reading data from the selected track. Because drives ordinarily employ error detecting codes, such a problem during a read operation generally involves only the delay associated with retry operations. A shock-caused off-track condition is a significant problem when it occurs during a write operation. If the off-tracking displacement involves a whole track shift, the data being written can replace needed data whereby the replaced data can no longer be retrieved from the disk.
The difficulty involved in solving the basic problems associated with shock-caused off-track conditions has increased because of the trend to increase the number of tracks per inch ("TPI"). A drive that has a large number of tracks per inch has a small radial distance between concentric tracks. Furthermore, because of the need to provide very short seek times, such a drive is designed so that the actuator assembly that supports the head has a very low moment of inertia. In other words, a head moved by shock need not travel very far to fly above the wrong track, and it can be so moved there very quickly.
A suitable actuator assembly for a state-of-the-art drive for a PC includes a voice coil motor ("VCM") and certain head-supporting elements. Such a drive also includes other elements that cooperate with the actuator assembly to define a servo system. The servo system causes the actuator assembly to move the head to fly above the desired track during track-seeking operations and to remain flying above the desired track during track-following operations. Such a servo system can include a dedicated servo surface and associated dedicated servo head; however, a dedicated servo system is generally not cost effective for drives that have at most two disks such as the small form-factor drives that are mass produced for the PC market. A more cost-effective approach involves a sampled data servo system often referred to as an "embedded" servo system. As explained more fully below, an embedded servo system, albeit generally advantageous, has characteristics that exacerbate the problem of preventing shock-caused off-track writing.
In a drive having an embedded servo system, each recording surface of each disk has arcuate locations termed "servo sectors" that are reserved for servo burst fields and that are not used for storing user data. During time spaced-apart intervals in which a servo burst field is being read, the servo system has access to the information needed to determine whether a positioning error exists. If such positioning error exists, the servo system effects an appropriate adjustment to cause the actuator assembly to move to reduce the positioning error.
User data is recorded in the arcuate locations extending between adjacent servo burst fields in track portions termed "data sectors". This user data includes sector identification information written in sector ID fields and also includes data generated by application programs. Sector IDs are read and verified to insure the head is reading or writing the correct sector. When the head is over an incorrect track, a sector ID error will cause a read or write operation to abort. If a shock causes the head to move off track during the time between the reading of servo burst fields, no indication thereof is provided to the servo system during such time. The movement from a desired track to an incorrect track can also occur after the head reads sector ID data indicating that the head is above the desired track. Thus, while a write operation is occurring, neither the servo system nor an incorrect sector ID can prevent a shock from causing a write operation on the wrong track.
A prior art approach to protecting data from the effects of such a shock is disclosed in U.S. Pat. No. 4,862,298. This approach involves the use in the disk drive of a tri-axial sensor and circuitry responsive thereto to produce three signals each representative of the magnitude of the shock acting along a respective one of three orthogonal lines. If any of the three signals exceeds a threshold amount (a "write-fault threshold"), writing by the drive can be turned off.
Such a tri-axial sensor, each of whose output signals represents only a linear component of the shock-caused motion along a respective one of three orthogonal lines, does not adequately provide for determining the rotational component of shock-caused motion. More generally, any signal produced by any similar sensor, which signal represents only a component of the shock-caused motion acting along a single line, does not provide a sufficient basis for determining the rotational component of the shock-caused motion. Unless such rotational component is determined, it is difficult if not impossible to set a write-fault threshold that consistently works properly. If a shock causes a linear acceleration along a line in a plane parallel to the disks and substantially spaced from the center of mass, a substantial angular acceleration can be produced by a relatively low level linear acceleration. In other words, the write-fault threshold set for linear acceleration would not be exceeded in situations in which it would be desirable for it to do so. On the other hand, a shock can cause a linear acceleration along a line in a plane parallel to the disks and through or close to the center of mass. In such cases, relatively little angular acceleration can be produced by a rather substantial linear acceleration. In this case, the linear acceleration may be sufficient to exceed the write-fault threshold and thereby result in a "false alarm," and writing of data is inhibited when it is not desirable to do so. Such "false alarms" cause "hesitations" during data write operations with a consequent degradation in the drive's write time performance parameters.
Additionally, using such a tri-axial sensor in a modern compact form factor disk drive involves placing an additional mechanical component inside a housing designed to minimize internal volume. Space must be provided to accommodate such a sensor in addition to the conventional mechanical and electromechanical components contained within a drive housing.
Accordingly, there is a need for a system which can provide a sufficient basis for determining the rotational component of a shock-caused motion and which can prevent a write operation from being carried out during an off-track condition. Such a system preferably minimizes "false alarms." Also, such a system preferably requires minimal, if any, additional space within a disk drive housing.